The invention relates to a circuit configuration for a fixed frequency blocking oscillator converter switching power supply, including a transformer having a primary winding in series with an electric switch in the circuit of a voltage source that outputs a direct voltage (input voltage) with a first algebraic sign, the switch being alternatingly switched on in a first operating phase (flux phase) and switched off in a second operating phase (blocking phase), and a secondary winding having a voltage (secondary winding voltage) from which a rectified output voltage is attained; and an integratable trigger circuit for pulse-width-modulated switching of the switch, having an oscillator emitting a voltage (oscillator signal) oscillating periodically between an upper and a lower peak value, and a pulse width modulator switching the switch as a function of the oscillator signal.
Fixed-frequency blocking oscillator converters or flyback converters with such circuits are known in a number of versions. For instance, reference is made to the monograph "Schaltnetzteile" [Switching Power Supplies] edited by J. Wustehube and published by Expert-Verlag in 1979, in particular chapter 3 thereof.
Blocking oscillator converters operating with fixed frequency generally require somewhat more expensive wiring than free-oscillating blocking oscillator converters. However, they are preferred whenever the instant of the current transition from the primary to the secondary side, which is the moment at which the highest-energy HF disturbances occur, is to be synchronized with some other operation. Such synchronization is recommended, for instance, for particularly high-power television switching power supplies or switch power packs, in which interference suppression is not readily successful. If the high-frequency noise pulses are shifted to the horizontal flyback of the TV set, then they remain invisible.
In a typical blocking oscillator converter having a fixed operating frequency, an integrated control circuit (trigger component) generates a synchronizable oscillator signal, forms a control signal from the deviation of the load voltage from a set-point value, and switches a switching transistor synchronously with the oscillator, in such a way that the collector peak current of the switching transistor is determined by the control signal. These functions are generally realized as follows (in this connection, reference is made to the publication TLE, December, 1988, No. 539, pp. 27-31, for instance): The oscillator receives two fixed switching thresholds; an applied synchronizing pulse shifts the upper switching threshold somewhat downward and as a result raises the oscillator frequency to the frequency of the synchronizing signal. The discharging pulse of the oscillator starts a trigger pulse for the switching transistor, upon which a base current proportional to its collector current is then imposed, in order to avoid oversaturation. The information on the collector current is attained from a measurement voltage, which is picked up either at an emitter resistor or at a resistor capacitor element connected parallel to the primary-side power element. If this measurement voltage exceeds a threshold value defined by the control signal, the trigger component terminates the trigger pulse.
In such a circuit, the collector current simulation requires a number of external components and a separate component connection. A further consideration is that the switching power supply can only be synchronized at considerable expense, for instance by including an extensive PLL switching circuit.